So I have been given a couple of questions from my tutor however I have no idea where to start, I'm not asking for the answers just a push in the right direction.

1)

270 Vibrio cholerae cells were inoculated into 10 ml of a liquid culture and incubated for 9 hours. At the end of this time the cells (in a volume of 100 microlitres) were spread onto agar plates and the colony forming units were enumerated after overnight incubation by counting the colonies on plates from different dilutions: e-5 = 643; e-6 = 75; e-7 = 6

Assuming that the cells multiplied by binary fission and the culture was in synchronized exponential growth throughout the incubation, how many full generations did the culture undergo and what was the generation time.

I guessed that you can only use one of the counts from the dilutions to perform the calculations and the number should be between 30 and 300 so I guess i'm using the 10^-6

2)

500 microlitres of a solution of a DNA fragment labelled with 32P with an initial activity of 0.4 kBq in 10 ml was needed for an experiment 9 days later. Calculate the radioactivity of the sample used in the experiment (T = half-life)

(I think I first need to work out the amount of radioactivity in the sample to be used in the experiment, but couldn't work that one out)

3)

Barium-140 has a half light of 13 days. How many decay half-lives does a sample of barium-140 undergo in 6 weeks?What fraction of the sample would remain after 6 weeks?

(Confusing question, I guessed barium-140 would go through 3.23 half-lives and that it would be around 1/8th of the sample left over, but exact number eludes me)

Last edited by Chemhalp on Wed Apr 23, 2008 7:59 pm, edited 1 time in total.

75 is the number of colonies and not the number of cells so you have to divide the number of colonies by the dilution factor which was 10^-6 and then x 100 to get the number of cells in 10ml. I got 7.5x10^9.